1. Field of the Invention
The present invention relates to two-port non-reciprocal circuit elements, and more specifically to a two-port non-reciprocal circuit element such as an isolator preferably for use in a microwave band.
2. Description of the Related Art
Non-reciprocal circuit elements such as isolators or circulators generally have a characteristic of transmitting a signal only in a predetermined specific direction and not transmitting a signal in the opposite direction. With the use of this characteristic, for example, an isolator is used in a transmission circuit unit of a wireless communication system such as a cellular phone.
A known two-port non-reciprocal circuit element of this type is described in Japanese Patent No. 4197032. The described two-port isolator includes a ferrite to which a direct-current magnetic field is applied by a permanent magnet, a first central electrode and a second central electrode which are disposed on the ferrite so as to be insulated from each other, a first capacitor electrically connected between an input port and an output port, a resistor electrically connected between the input port and the output port, a second capacitor electrically connected between the output port and a ground port, an input terminal, and an output terminal. An impedance matching capacitor is electrically connected at least between the input port and the input terminal or between the output port and the output terminal, and a coupling capacitor is electrically connected between the input terminal and the output terminal.
The coupling capacitor is configured to adjust an insertion loss characteristic and an isolation characteristic using the trade-off between them. However, the coupling capacitor has an impedance that decreases as the operating frequency increases, and thus, at a high operating frequency, the input port and the output port are substantially directly coupled to each other in a harmonic frequency band, resulting in it being difficult to obtain a desired harmonic attenuation. In the future, it is expected to implement a wireless communication system for high-frequency applications, and the problem described above is considered to become serious. Adding a trap circuit enables an improvement in harmonic attenuation, whereas the complexity of a structure or a circuit increases. There is also a problem of degradation in insertion loss.